After cutting timber into boards, the boards must be reduced in moisture content before they can be used commercially. During this period of drying, the surfaces of the boards must, of course, dry and therefore shrink before the interior of the board is dried. Unless the surface drying is carefully controlled, the resultant shrinkage of the surface can result in splitting or checking of the surface which will severely reduce the value of the lumber. Control of the moisture content of the surface is normally accomplished by either controlling the humidity of the air as in kiln drying, or by air drying in the outdoors during suitable seasons. Also, as disclosed by Leischner in U.S. Pat. No. 2,679,111, the control of surface moisture content can be accomplished by placing a thin barrier or sheet of suitable material between the wood surface and the circulating air in order to maintain a layer of moisture and thus protect the wood surface from the drying air.
Normally, lumber is air dried in stacks with each single layer or course of boards being separated from the adjacent layers by spacers called stickers which allow air circulation across the surfaces of the boards. The stickers, which are wood, are usually about 0.75 inches thick by 1.25 to 1.50 inches wide, are laid at right angles to the boards and are of a length that is determined by the width of the layer of boards. Except where they are in contact with the stickers, the surfaces of the boards are exposed to the drying air and may quickly dry to very near the equilibrium moisture content (hereinafter sometimes referred to as EMC) of the air. In kiln drying, temperature and humidity are automatically controlled so that the EMC of the air in the kiln is initially at a high level. The EMC is then gradually reduced as the drying process proceeds which allows for the gradual shrinkage of the surfaces of the boards and thereby avoids the surface checking that results if the surfaces are dried too quickly.
Wood which is to be used in the manufacturing of furniture and for decorative purposes must be free of surface checks and other drying defects. Therefore, these species of wood, particularly those which are commonly referred to as "refractory hardwoods" because of their tendency to check easily, must be dried very slowly and carefully in order to maintain their value. Because kiln drying requires vast expenditures of energy, air drying in stacks utilizing stickers is often used until the wood has dried to the point where it can be safely put in the kiln and quickly brought to the final desired level of dryness.
However, said air drying leaves the wood at the mercy of the weather. Refractory woods, because of their slow rate of moisture diffusion, are relatively slow in drying. Thus, the moisture from the interior of the board is slow in reaching the surfaces of the board. As a result, the surfaces quickly approach the EMC level of the drying air, and a period of severe drying weather can result in disastrous losses of refractory woods in a single drying yard. To avoid the possibility of a sustained period of severe drying weather, refractory woods are often put in the drying yard in late fall or winter. This allows for substantial drying to occur prior to the severe drying weather of summer.
Leischner in U.S. Pat. No. 2,679,111 discloses another method of drying lumber to avoid checking and splitting of the surface. Leischner places porous material with a moisture permeability less than the lumber around the piece of lumber to be dried. This provides for control of the moisture content at the surfaces of the lumber with the concurrent control of shrinkage. But in order to accomplish this method of controlled drying Leischner provides space for the circulating air and support for the entire structure by using a heavy bulky metal apparatus consisting of two perforated metal sheets which are kept spaced apart by a third corrugated sheet welded to the two perforated sheets. The purpose of this structure is to provide both support for the porous covering sheets and space between the covering sheets for circulation of the drying air. Because of this structure, which is inherently unsuitable and impractical for use in drying yards, the apparatus disclosed by Leischner is not used by drying yards. Instead, as discussed above, lumber is air dried in stacks with means for spacing provided to allow circulation of air between the layers but continually present is the danger of checking and splitting which often results from the uncontrolled air drying of wood.
In order to overcome all of these problems, an article has been developed to facilitate the air drying of refractory hardwood with minimum drying defects even in severe drying weather. Hygroscopic sheets, herein termed either face sheets or facing sheets, are glued, nailed, stapled, or otherwise attached to spacing means to form an integral article. These articles hereinafter sometimes referred to as pallets are then alternately layered with the boards of lumber forming stacks. With the lumber alternately layered in this manner with the pallets, the face sheets separate the board surfaces from the drying air which instead of contacting the surfaces of the boards directly now circulates between the face sheets. The face sheets may be of any suitable hygroscopic material. One readily available material is 3-ply plywood. The moisture permeability of the face sheets may be predetermined through selection of the species and thickness of the wood veneers and the nature of the glue used to make the plywood face sheets. Thus the pallet can be engineered to provide the degree of protection deemed necessary for both the particular wood species that is to be dried and the severity of drying weather that is expected in the given locality.